1. Field of the Invention
The present invention relates to a liquid crystal display device, a light source device, and a light source control method that blinks illumination light on and off for gradation display and non-gradation display which are performed periodically.
2. Description of the Related Art
In recent years, a liquid crystal display panel of the OCB mode has received attention because it has a good response of liquid crystal molecules required for display of moving images. With this liquid crystal display panel, the liquid crystal molecules are in a splay alignment before supply of power. This splay alignment is a state where the liquid crystal molecules are virtually laid down, and is transitioned to a bend alignment for display operation, upon supply of power. The splay alignment is more stable than the bend alignment in terms of energy. The reverse transition to the splay alignment tends to occur if a voltage-non-applied state, or a voltage applied state of a voltage not greater than a level at which energy of the splay alignment is balanced with energy of the bend alignment, continues for a long time. Conventionally, as a measure for preventing the reverse transition, a driving method has been proposed which periodically applies a large voltage to all liquid crystal pixels (see, for example, Japanese Patent Applications Nos. 2000-214827 and 2002-107695). With a normally white liquid crystal display panel, since the above-mentioned voltage corresponds to a pixel voltage that provides black display, the driving method is referred to as black insertion driving.
The liquid crystal display panel is a hold type display device that causes all pixels to hold pixel voltages for each frame period, which is the display image updating cycle. With the black insertion driving, for example, pixel voltages for gradation display are applied to all the pixels in units of one row in the first half of one frame period (one vertical scanning period) and pixel voltages for black insertion are applied to all the pixels in units of one row in the second half of the same frame period. Each pixel holds a pixel voltage for gradation display until application of a pixel voltage for black insertion, and then holds the pixel voltage for black insertion until application of a pixel voltage for gradation display. Here, the ratio of the holding period of pixel voltage for black insertion to the holding period of pixel voltage for gradation display is referred to as the black insertion ratio.
With the hold type display device, it is difficult to display the movement of an object smoothly due to retinal persistence occurring on viewer's vision in a moving image display. The black insertion driving causes the retinal persistence to be cleared by discrete pseudo-impulse response of luminance, and is therefore effective in improving the visibility of moving images that lowers according to the viewer's vision. However, the black display state obtained by the black insertion driving does not provide perfect black as obtained when the backlight, which is an illumination light source, is turned off. For this reason, to obtain better visibility of moving images the employment of blinking driving to blink the backlight on and off has been studied. Incidentally, the black insertion ratio required to prevent the reverse transition is of the order of 2.5%. The visibility of moving images improves as the black insertion ratio increases.
With use of blinking driving, the ratio of the backlight lighting period to the blinking cycle, which is normally one vertical scanning period, is usable to adjust the brightness of the entire liquid crystal display panel. Conventionally, the backlight lighting period is controlled by the pulse duration (pulse width) of a pulse width modulation (PWM) signal, and lighting control to limit the luminance of the backlight is performed by reducing the pulse duration. However, the difference in optical response between the liquid crystal pixels and the backlight results in a problem that the contrast ratio considerably drops as the pulse duration decreases.